This invention relates to semiconductor devices, and particularly to silicon on insulator devices having the body-tied-to-source and methods of fabricating such devices.
Demands for increased performance, functionality and manufacturing economy for integrated circuits have resulted in extreme integration density while also increasing the number of circuits and devices that can be formed on a wafer by a single sequence of processes. Scaling of devices to small sizes has restricted operating margins and has necessitated an increased uniformity of electrical characteristics of semiconductor devices formed on a wafer.
To satisfy this latter criterion, silicon on insulator (SOI) devices have been employed to exploit the improved quality of single crystalline silicon through an active layer thereof formed on an insulator residing upon a bulk silicon “handling” substrate. Various active layers can be formed upon the insulator that are electrically isolated from each other. As a result, SOI structures have reduced parasitic capacitances relative to bulk devices. The improved quality of the semiconductor material of the active SOI layers allows transistors and other devices to be scaled to extremely small sizes with good uniformity of electrical properties.
Unfortunately, the existence of the insulator layer (also referred to a “buried oxide layer” or “BOX”) presents a problem known in the art as the “floating body effect” in SOI MOSFET (metal-on-silicon field effect transistor) devices. Those devices include a neutral floating body in the semiconductor layer that is spaced from the gate conductor by the gate dielectric. The neutral floating body is electrically isolated by source/drain and halo regions that form oppositely poled diode junctions at the ends of the transistor conduction channel and floating body. The insulator layer in the substrate completes insulation of the conduction channel and thus prevents discharge of any charge that may develop in the floating body. Charge injection into the neutral body when the transistor is not conducting develops voltages in the conduction channel in accordance with the source and drain diode characteristics.
The floating body effect is induced by the excess carriers generated by hot electrons near the strongly filed gradient drain region, resulting in enhanced body potential in SOI devices. This effect, in turn, induces a reduction in the threshold voltage VT, causing a kink in the output characteristics. The voltage developed due to charge collection in the transistor conduction channel has the effect of altering the switching threshold of the transistor. This effect, in turn, alters the signal timing and signal propagation speed since MOSFET's have a finite slew rate and the rise and fall time of signals is not instantaneous even when the threshold voltage is not uniform across a given integrated circuit. SOI switching circuits, in particular, suffer from severe dynamic floating body effects such as hysteresis and history effects. The onset of the kink effect in SOI switching circuits strongly depends on operating frequency and produces Lorentzian-like noise overshoot and harmonic distortion. Soft error issues are also more serious in SOI MOSFET devices.
In order to limit the charge that builds up in the floating body, body contacts and body-tied-to-source structures have been incorporated in SOI MOSFET devices. However, current body-tied-to-source structures can require multiple processing steps and impose a layout penalty relative to non-body contacted devices.